1. Field of the Invention
This invention relates to a disk storage system applied to a hard disk drive, for example, and using a disk on which servo burst data for controlling the positioning of a head for recording and reproducing data is recorded as a data recording medium.
2. Description of the Related Art
Conventionally, a disk storage system such as a hard disk drive (HDD) is used as a device which uses a disk as a data recording medium, records data on the disk by use of a head and reproduces data from the disk.
In a small HDD, a sector servo type head positioning control apparatus for controlling the positioning of the head on a target track (target cylinder) on the disk according to servo information previously recorded on the disk is provided. The head positioning control mainly includes the seeking operation for moving the head to a target track and the track following control operation (which is a positioning operation in a narrow sense) for positioning the head in a range of the target track or at the center thereof.
In the sector servo system, servo information previously recorded on servo sectors arranged at preset intervals on each of the concentric tracks on the disk is used. Each track has a preset number of data sectors arranged between the servo sectors and user data is recorded in the data sector. The servo information mainly includes cylinder data (track address code) used for the seeking operation and servo burst data relating to this invention.
The servo burst data is constructed by a plurality of burst patterns for deriving a positional error (position data) of the head in a range of the target head or in a range to the adjacent track after the head is moved to a position near the target track by the seeking operation. The CPU of the head positioning control apparatus converts the amplitude of a position signal waveform obtained when the head reads the burst pattern into digital data and effects the processing operation for deriving the positional error by using the digital data.
As shown in FIG. 7A, the servo burst data is constructed by first to fourth burst patterns A, B, C, D recorded in part of the servo sector. Among them, the first and second burst patterns A and B adjacent to each other make a pair of patterns having different phases and are used as a set of first phase. Further, the third and fourth burst patterns C and D adjacent to each other make a pair of patterns having different phases and are used as a set of second phase.
The recording width of each of the burst patterns A, B, C, D is the same as one track width. As shown in FIG. 7A, the burst patterns A and B are alternately arranged in the radial direction of the disk with the center TC of the track (N) set as a boundary. The burst patterns C and D are alternately arranged with the boundary between the adjacent tracks set as a reference.
In the head positioning control, the seeking operation is effected by use of cylinder data read out by the head and the head is moved to a specified target track. In this case, it is supposed that an MR head which is a read head 20 to be described later has been moved to a target track N as shown in FIG. 7A. In the head positioning control, the seeking operation is changed over to the track following control operation. By the track following control operation, the read head 20 is positioned at the center TC of the target track N.
In the track following control operation, the read head 20 reads out position signals from the burst patterns A, B, C, D. The head positioning control apparatus samples and holds the amplitude values of the burst patterns A, B, C, D, converts the amplitude values into digital data by use of an A/D converter, and then outputs the digital data to the CPU. The CPU effects the arithmetic operation "(A-B)/(A+B)" by using the amplitude values A and B corresponding to the first and second burst patterns A and B to derive a positional error of the read head 20. Likewise, the CPU effects the arithmetic operation "(C-D)/(C+D)" by using the amplitude values C and D corresponding to the third and fourth burst patterns C and D to derive a positional error of the read head 20.
In this case, the results of the arithmetic operations for positional errors by the CPU can be expressed as shown in FIGS. 7B and 7C. That is, when the read head 20 is positioned on the center TC of the track N as shown in FIG. 7A, the positional error is "0". If the read head 20 is set in position shifted in a direction towards the burst pattern A, the amount of positional error changes towards "+1" as shown in FIG. 7B. On the other hand, if the read head 20 is set in position shifted in a direction towards the burst pattern B, the amount of positional error changes towards "-1" as shown in FIG. 7B. If the read head 20 is positioned in the range of the burst pattern A or B, the amount of positional error is fixedly set at "+1" or "-1", respectively. Therefore, if the read head 20 is deviated from the track center TC and is set closer to the boundary with the adjacent track (N+1 or N-1), an area (non-sensible area) NS in which the positional error of the read head 20 cannot be detected occurs. Therefore, the CPU derives the positional error of the read head 20 with the boundary between the adjacent tracks set as a reference by using the amplitude values C and D corresponding to the third and fourth burst patterns C and D.
Thus, in the conventional head positioning control apparatus, a method using the linear sections LS of the positional error characteristics based on the two-phase burst patterns A, B and C, D as shown in FIGS. 7B, 7C to detect the positional error of the head 20 in the entire track range is used.
Recently, in the HDD, a reproduction/recording separation type head using an MR (magnetroresistive) head as a read head has received much attention in order to realize the high-density recording. As shown in FIG. 8, a reproduction/recording separation type head 2 has a composite head structure in which an MR head used as the read head 20 and a write head 21 using an induction type thin-film head are mounted on a slider. In other words, the reproduction/recording separation type head 2 has a structure in which two gaps including a read gap of the MR head and a write gap of the induction type head are formed on one slider.
In the reproduction/recording separation type head 2, the head width of the write head 21 is substantially equal to the track width and the head width of the read head 20 is approximately equal to half the track width. At the time of data reading operation, the CPU positions the read head 20 at the center TC of the track based on servo burst data read out by the read head 20 as described before. On the other hand, at the time of data writing operation, the CPU positions the write head 21 at the center TC of the track, that is, in the range of the track. At this time, the positioning control for the write head 21 is effected based on servo burst data read out by the read head 20. In the reproduction/recording separation type head 2, the centers of the gaps of the read head 20 and write head 21 are deviated from each other for the structural reason. Therefore, at the time of data writing operation, the read head 20 reads out burst data not in the track center TC but in position (off-track position) slightly deviated from the track center TC as shown in FIG. 8. As a result, at the time of data writing operation, the read head 20 cannot correctly read out the burst patterns A to D in some cases, and in this case, it becomes impossible to precisely position the write head 21 at the center TC of a target track.
Further, the head width of the read head 20 is smaller than the track width as described before. Therefore, at the time of data reading operation, the influence of fringe noise occurring in the boundary area between the adjacent tracks can be avoided. However, if the head width is relatively small, the range of the linear section LS for detecting the positional error in the positional error characteristic becomes relatively narrow and the non-sensible area NS in which the positional error cannot be detected is enlarged to a range indicated by broken lines. If the non-sensible area NS is large, a range in which the position of the head lying in the track range cannot be detected becomes large. Therefore, it takes a long time for the settling operation for precisely positioning and holding the head 20 at the track center TC in the head positioning control and the positioning precision is lowered.